Many heat exchangers in use today, as, for example, vehicular radiators, oil coolers, and charge air coolers, are based on a construction that includes two spaced, generally parallel headers which are interconnected by a plurality of spaced, parallel, flattened tubes. Located between the tubes are thin, serpentine fins. In the usual case, the side most tubes are located just inwardly of side plates on the heat exchanger and serpentine fins are located between those side most tubes and the adjacent side plate.
The side plates are typically, but not always, connected to the headers to provide structural integrity. They also play an important role during the manufacturing process, particularly when the heat exchanger is made of aluminum and components are brazed together or when the heat exchanger is made of other materials and some sort of high temperature process is involved in the assembly process.
More particularly, conventional assembly techniques involve the use of a fixture which holds a sandwiched construction of alternating tubes and serpentine fins. The outside of the sandwich, that is the outer layers which eventually become the sides of the heat exchanger core, is typically provided with side plates whose ends are typically connected mechanically to the headers. Pressure is applied against the side plates to assure good contact between the serpentine fins and the tubes during a joining process such as brazing to assure that the fins are solidly bonded to the tubes to maximize heat transfer at their points of contact. If this is not done, air gaps may be located between some of the crests of the fins and the adjacent tube which adversely affect the rate of heat transfer and durability, such as the ability to resist pressure induced fatigue and to withstand elevated pressures.
At the same time, when the heat exchanger is in use, even though the side plates may be of the same material as the tubes, because a heat exchange fluid is not flowing through the side plates but is flowing through the tubes, the tubes will typically be at a higher temperature than the side plates, at least initially during the start up of a heat exchange operation.
This in turn results in high thermal stresses in the tubes and headers. Expansion of the tubes due to relatively high temperatures tends to push the headers apart while the side plates, at a lower temperature, tend to hold them together at the sides of the core. All too frequently, this severe thermal stress in the heat exchanger assembly results in fracture or the formation of leakage openings near the tube to header joints which either requires repair or the replacement of the heat exchanger.
It has been proposed to avoid this problem, after complete assembly of the heat exchanger, by sawing through the side plates at some location intermediate the ends thereof so that thermal expansion of the tubes is accommodated by the side plates, now in multiple sections, which may move relative to one another at the saw cut. However, this solution adds an additional operation to the fabrication process and consequently is economically undesirable.
Another approach is to construct the side plate so that it breaks when it is put in tension by positive stresses caused by a differential thermal expansion, such as shown in U.S. Pat. No. 6,412,547, issued Jul. 2, 2002 and naming Nicholas R. Siler as the inventor. This approach eliminates the need for an additional operation such as saw cutting. However, in addition to the above positive stresses caused by expansion, heat exchangers may also undergo negative stresses or compression. Negative stresses may be caused by thermal expansion and contraction of the heat exchanger itself as well as the thermal expansion and contraction of external components connected to the heat exchanger which may cause the heat exchanger to compress. The above solution shown in the U.S. Pat. No. 6,412,547 patent does not provide for compression of the side plate caused by negative stresses.